The present invention relates generally to spread spectrum communications systems, processes and systems that use spread spectrum communication, and, in particular embodiments to such systems, processes and devices which improve the performance of spread spectrum communications systems in the presence of adverse signal conditions.
It is likely that the first navigational system used by man was the sun. It rose in the east and set in the west and as long as travel occurred in daylight hours general direction could be obtained from the sun""s position in the sky. With the advent of commerce on the seas it became necessary to ascertain direction at night, and so stellar navigation was born. In order to increase accuracy, stellar and solar navigation techniques were improved and augmented through the use of maps, charts, and instruments such as the astrolabe and compass. Even augmented by such instruments, stellar and solar navigation was error prone and traveling from point A to point B was still, to some extent, a matter of trial and error.
With the advent of radio, and particularly powerful commercial radio stations land based radio direction finding (RDF) came into being. The principle behind RDF is relatively simple. A navigator can tune to a radio station using a directional antenna to find the directional bearing of the radio station. The navigator can then could tune to a second radio station and find the bearing of that station. By knowing the bearing and map location of both stations the navigator""s position can be calculated.
Continuing advances in long distance air travel necessitated the ability to guide aircraft accurately. RDF was used to satisfy this requirement and land based beacons were established for the purpose of navigation. These beacons quickly became indispensable to all aviation and to ships as well.
The Global Positioning System (GPS) is also based on radio navigation, a difference being that the beacons are no longer stationary and are no longer land based. The GPS system is a satellite based navigation system having a network of 24 satellites, plus on orbit spares, orbiting the earth 11,000 nautical miles in space, in six evenly distributed orbits. Each satellite orbits the earth every twelve hours.
A prime function of the GPS satellites is to serve as a clock. Each satellite derives its signals from an on board 10.23 MHz Cesium atomic clock. Each satellite transmits a spread spectrum signal with it""s own individual pseudo noise (PN) code. By transmitting several signals over the same spectrum using distinctly different PN coding sequences the satellites may share the same bandwidth without interfering with each other. The code used in the GPS system is 1023 bits long and is sent at a rate of 1.023 megabits per second, yielding a time mark, sometimes called a xe2x80x9cchipxe2x80x9d approximately once every micro-second. The sequence repeats once every millisecond and is called the course acquisition code (C/A code). Every 20th cycle the code can change phase and is used to encode a 1500 bit long message which contains an xe2x80x9calmanacxe2x80x9d containing data on all the other satellites.
There are 32 PN codes designated by the GPS authority. Twenty four of them belong to current satellites in orbit, the 25th PN code is designated as assigned to any satellite. The remaining codes are spare codes which may be used in new satellites to replace old or failing units. A GPS receiver may, using the different PN sequences, search the signal spectrum looking for a match. If the GPS receiver finds a match, then it has identified the satellite which has generated the signal.
Ground based GPS receivers may use a variant of radio direction finding (RDF) methodology, called triangulation, in order to determine the position of the ground based GPS receiver. The GPS position determination is different from the RDF technology in that the radio beacons are no longer stationary they are satellites moving through space at a speed of about 1.8 miles per second as they orbit the earth. By being space based, the GPS system can be used to establish the position of virtually any point on Earth using methods such as triangulation.
The triangulation method depends on the GPS receiving unit obtaining a time signal from a satellite. By knowing the actual time and comparing it to the time that is received from the satellite the receiver, the distance to the satellite can be calculated. If, for example, the GPS satellite is 12,000 miles from the receiver then the receiver must be somewhere on the location sphere defined by the radius of 12,000 from that satellite. If the GPS receiver then ascertains the position of a second satellite it can calculate the receiver""s location based on a location sphere around the second satellite. The two sphere""s intersect and form a circle, and so the GPS receiver must be located somewhere within that location circle. By ascertaining the distance to a third satellite the GPS receiver can project a location sphere around the third satellite. The third satellite""s location sphere will then intersect the location circle produced by the intersection of the location spheres of the first two satellites at just two points. By determining the location sphere of one more satellite, whose location sphere will intersect one of the two possible location points, the precise position of the GPS receiver is determined. As a consequence, the exact time may also be determined, because there is only one time offset that can account for the positions of all the satellites. The triangulation method may yield positional accuracy on the order of 30 meters, however the accuracy of GPS position determination may be degraded due to signal strength and multipath reflections.
As many as 11 satellites may be received by a GPS receiver at one time. In certain environments such as a canyon, some satellites may be blocked out, and the GPS position determining system may depend for position information on satellites that have weaker signal strengths, such as satellites near the horizon. In other cases overhead foliage may reduce the signal strength that is received by the GPS receiver unit. In either case the signal strength may be so reduced as to make acquisition of enough satellites to determine position difficult. In such cases the GPS receiver may take a number of attempts to lock onto the satellites and increased time to lock onto signals suitable for position determination, or it may not be able to lock on to the signals at all.
In the case of multipath reflections, a signal may be reflected from a structure, or even the ground, so that the signal""s path to the receiver is indirect. An indirect path is longer than a direct path and will make the time the signal has to travel to the receiver longer and the distance to the satellite will appear to be farther away as a result. The resulting position calculated by the receiver may contain an error.
There are multiple ways of using radio spectrum to communicate. For example, in frequency division multiple access (FDMA) the frequency band is divided into a series of frequency slots and different transmitters are allotted different frequency slots.
n time division multiple access (TDMA) systems in which the time that each transmitter may broadcast is limited to a time slot, such that transmitters transmit their messages one after another, only transmitting during their allotted period. With TDMA, the frequency upon which each transmitter transmits may be a constant frequency or may be continuously changing (frequency hopping).
A third way of allotting the radio spectrum to multiple users is through the use of code division multiple access (CDMA) also known as spread spectrum. In CDMA all the users transmit on the same frequency band all of the time. Each user has a dedicated code that is used to separate that user""s transmission from all others. This code is commonly referred to as a spreading code, because it spreads the information across the band. The code is also commonly referred to as a Pseudo Noise or PN code. In a CDMA transmission, each bit of transmitted data is replaced by that particular user""s spreading code if the data to be transmitted is a xe2x80x9c1xe2x80x9d, and is replaced by the inverse of the spreading, code if the data to be transmitted is xe2x80x9c0xe2x80x9d.
To decode the transmission at the receiver it is necessary to xe2x80x9cdespreadxe2x80x9d the code. The despreading process takes the incoming signal and multiplies it by the spreading code and sums the result. This process is commonly known as correlation, and it is commonly said that the signal is correlated with the PN code. The result of the despreading process is that the original data may be separated from all the other transmissions, and the original signal may be recovered. A property of the PN codes that are used in CDMA systems is that the presence of one spread spectrum code does not change the result of the decoding of another code. The property that one code does not interfere with the presence of another code is often referred to as orthogonality, and codes which possess this property are said to be orthogonal.
Although the term CDMA is used widely to describe a type of telephone communications the term spread spectrum may also be applied. Those terms will be used interchangeably herein.
The process of extracting data from a CDMA signal is commonly known by many terms such as correlating, decoding, and despreading. Those terms will be used interchangeably herein.
The codes used by a spread spectrum system are commonly referred to by a variety of terms including, but not limited to, PN (Pseudo Noise) codes, PR (Pseudo Random) codes, spreading code, despreading code, and orthogonal code Those terms will be used interchangeably herein.
It is because the transmission bandwidth is large compared with the data bandwidth that CDMA is often referred to as spread spectrum. Spread spectrum has a number of benefits. One benefit being that because the data transmitted is spread across the spectrum, spread spectrum can tolerate interference better than some other transmission protocol. Another benefit is that messages can be transmitted with low power and still be decoded.
The Global Positioning System uses spread spectrum technology to convey its data to ground units. The use of spread spectrum is especially advantageous in the GPS systems. Spread spectrum technology enables GPS receivers to operate on a single frequency, thus saving the additional electronics needed to switch and tune other bands if multiple frequencies were used. Spread Spectrum also can minimize the power consumption requirements of the GPS system, for example to require 50 watts or less and tolerate substantial interference.
Although the GPS system is available widely, there are conditions which can degrade its performance or block its use. Improvements in the reception of GPS signals are constantly being sought. There is a need for greater sensitivity in receiving GPS signals to improve the performance of ground based receivers.
CDMA (Code Division Multiple Access) is used as an alternative to FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) in portable phone systems. There has been widespread debate over the superiority of TDMA versus CDMA. Both technologies continually vie for market acceptance and market share, and because of this competition improvements in each are constantly being sought. There is therefore a need in the art for improvement of the sensitivity of CDMA portable phone systems.
Accordingly, preferred embodiments of the present invention are directed to systems which incorporate spread spectrum technology. GPS receivers, cordless and portable telephones are examples of systems which may employ spread spectrum technology, and are used herein to illustrate preferred embodiments of the invention.
Spread spectrum technology can be advantageous in applications where signals need to be transmitted with low power. This is""so because the spreading process introduces redundancy into the signal being transmitted by replacing each data bit by a series of bits commonly called Pseudo Noise, or PN codes. This redundancy enables the signal to be decoded even though parts of it may be obscured by noise.
To receive the data a spread spectrum receiver must determine that the PN code is present. To determine that a PN code is present in a received signal, the received signal must be compared, bit by bit, to the PN code. This process of comparing a signal bit by bit to a PN code is commonly known as correlation. A perfect match will indicate the presence of the signal. Less than a perfect match, however, may also indicate the presence of the signal. This is so because noise may disturb the bit pattern of the signal broadcast, with the result that not all bits of the signal match the code. One of the advantages of spread spectrum is that the signal is redundant so that, even if a number of bits are destroyed by noise, the remaining bits may be detected and the signal decoded.
To determine that a particular spread spectrum is present, the received signal must be compared, bit by bit, to the PN code, and the results of the comparison examined. If the result is a 100% match between the PN code and the signal received a signal containing the code is probably present. If, however, there is less than a 100% match the signal may still be present. To examine for the presence of the signal a process commonly known as hypothesis testing is employed. Hypothesis testing examines a signal for a certain percentage match. If that percentage match, between the signal and the PN code, is met or exceeded, the hypothesis testing proceeds to test for a greater percentage threshold. The process of testing for increasingly higher percentage match thresholds, between the signal and the PN code, terminates when a final threshold is passed and it is determined that the signal is present. The hypothesis testing process may also terminate if successive attempts are not successful in passing the percentage matching thresholds. If a certain number of successive attempts fail to pass the successive thresholds, of the hypothesis tester, it is determined that the signal, containing the PN code, is not present. The hypotheses testing process can insure with a high probability that a signal is present, even if a 100% match is never obtained. The hypotheses testing process helps to eliminate false positive matches, and also helps to prevent false negatives (erroneous indications that the particular PN code is not present).
The hypothesis testing process has a weakness in that it may take a long time and multiple attempts to determine that a weak signal is present. Multiple attempts at matching the PN code to the signal may be necessary because, in case of weak signal conditions, random noise can easily interfere with the signal, destroying the bit pattern. The process of hypothesis testing depends on multiple samplings of the signal, thereby, on average, negating the effect of random noise. The hypothesis testing process also has a weakness in that reflected, or multipath, signals may be strong enough to cause a false indication.
Embodiments of the disclosure provide several improvements upon the process of hypothesis testing and of spread spectrum signal processing in general. One preferred embodiment measures the signal strength of the incoming spread spectrum signal and adjusts the hypotheses testing levels depending on the level of received signal strength. Decreasing the threshold levels, in the case where a spread spectrum signal is weak, will decrease the time that will be needed to identify that a signal bearing a certain PN code is present. Increasing the threshold levels, when the signal is strong, will help prevent multipath signals from being falsely identified as being the broadcast signal, bearing the PN code, sought.
In addition, various embodiments utilize both old and novel methods of determining signal strength. New methods, employed in several embodiments of the disclosure, use a PN code not present in the spread spectrum signal being decoded, to determine the signal strength. The code, which is not present in the spread spectrum signal received, is correlated with the spread spectrum signal. The result of the correlation with the unused code is indicative of the signal strength. In a GPS embodiment of the disclosure the 25th GPS code is correlated with the incoming signal. The 25th GPS code is one which has been identified by the GPS authority as not being present in any GPS satellite. Correlation of the GPS signal with this code yields a result that is indicative of the strength of the spread spectrum signal.